Technical Field
The present disclosure relates to an actuator and pre-compression magnetic force calibration method thereof, especially relates to a springless voice coil actuator and pre-compression magnetic force calibration method thereof.
Description of Related Art
As the development of technology, pixels of a camera module for mobile phones are increasingly higher to achieve, so far, tens of megapixels of resolution. However, the quality of the photos taken by the camera module in a mobile phone is not only depended on the higher resolution, but also on the auto-focusing lens module of the camera module. Recently, various actuators for the auto-focusing lens module can be obtained, such as voice coil motor (VCM), stepping motor, piezoelectric motor, liquid lens, enhanced crystal lens and polymer deformable membrane.
The aforementioned actuators for the auto-focusing lens modules are normally classified into a rotationally-driven type and a transnationally-driven type.
The rotationally-driven type actuator (e.g. stepping motor) mainly features that such actuator itself can drive the lens module to move and be in position with no need of extra electric power. However, the rotational movement of the stepping motor should be transformed to a linear movement by an extra mechanism in order to drive the lens module to move parallel to the optical axis. For obtaining a precision control, the structure of the extra mechanism is complicated and the size becomes larger, which is not favorable to the miniaturization of the lens module.
In this regard, the transnationally-driven type actuator has reached in the market for obtaining compact size and simple structure for the lens module. The main advantage of the transnationally-driven type actuator (e.g. voice coil motor, piezoelectric motor or liquid lens) is its direct control on the displacement of the lens module. Thus, the transnationally-driven type actuators have been in active development. Among such transnationally-driven type actuators, the voice coil motor is the one widely used owing to the advantages such as low cost, compact size, energy consumption, precision position and rapid response.
The main structure of the voice coil motor (actuator) is to dispose a coil into a magnetic circuit formed by a permanent-magnetism. According to Fleming's left-hand rule, when a current is exerted to the coil, the coil is interactive with a magnetic field of the permanent-magnetism and forming a pushing force. The pushing force pushes the lens module to generate a displacement thereby forming an auto-focusing or an auto-zooming. During the process of auto-focusing, a pre-compression force having direction opposite to the pushing force is necessary, in an open-loop control, for balancing and precisely positioning the lens module into a correct position.
Conventional voice coil actuator can be classified to an open-loop control and a close-loop control. In the open-loop control, an elastic force of a spring is utilized as a pre-compression force; the pre-compression force is balanced with the pushing force for repeatedly positioning the lens module. In the close-loop control, an extra position measurement component (e.g. Hall element) is utilized for generating a position feedback signal, and the lens module is positioned in accordance with the position feedback signal. The close-loop controlled actuator has disadvantage on high cost from extra-added position measurement component and complicated circuit design. Therefore, the open-loop controlled voice coil actuator is more popular in the field. However, there are some deficiencies in the open-loop controlled voice coil. First, the open-loop controlled voice coil actuator conventionally utilizes the elastic force of the spring, which is very complicated in structure, as the pre-compression force, and thus the manufacturing process also becomes more complicated and incurs the higher manufacturing cost. Second, the elastic force of the spring is not easy to be designed, and the pre-compression force is usually increased too fast. As a result, a larger current is required for balancing owing to the rigidity must be large enough to overcome the runout of the optical axis of the lens module. Therefore, the energy consumption is large, which does not meet the energy saving requirement.